[PATCH] readahead: add notes on readahead size

[Wu Fengguang <fengguang.wu@intel.com>]

> readahead: limit read-ahead size for small memory systems
> 
> When lifting the default readahead size from 128KB to 512KB,
> make sure it won't add memory pressure to small memory systems.

btw, I wrote some comments to summarize the now complex readahead size
rules..

==
readahead: add notes on readahead size

Basically, currently the default max readahead size
- is 512k
- is boot time configurable with "readahead="
and is auto scaled down:
- for small devices
- for small memory systems (read-around size alone)

CC: Matt Mackall <mpm@selenic.com>
CC: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/readahead.c |   22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

--- linux.orig/mm/readahead.c	2010-02-26 10:11:41.000000000 +0800
+++ linux/mm/readahead.c	2010-02-26 10:11:55.000000000 +0800
@@ -7,6 +7,28 @@
  *		Initial version.
  */
 
+/*
+ * Notes on readahead size.
+ *
+ * The default max readahead size is VM_MAX_READAHEAD=512k,
+ * which can be changed by user with boot time parameter "readahead="
+ * or runtime interface "/sys/devices/virtual/bdi/default/read_ahead_kb".
+ * The latter normally only takes effect in future for hot added devices.
+ *
+ * The effective max readahead size for each block device can be accessed with
+ * 1) the `blockdev` command
+ * 2) /sys/block/sda/queue/read_ahead_kb
+ * 3) /sys/devices/virtual/bdi/$(env stat -c '%t:%T' /dev/sda)/read_ahead_kb
+ *
+ * They are typically initialized with the global default size, however may be
+ * auto scaled down for small devices in add_disk(). NFS, software RAID, btrfs
+ * etc. have special rules to setup their default readahead size.
+ *
+ * The mmap read-around size typically equals with readahead size, with an
+ * extra limit proportional to system memory size.  For example, a 64MB box
+ * will have a 64KB read-around size limit, 128MB mem => 128KB limit, etc.
+ */
+
 #include <linux/kernel.h>
 #include <linux/fs.h>
 #include <linux/memcontrol.h>